There are many factors that affect the amount of backlash in a drive train, and in precision applications, compensations must be made to minimize and/or eliminate that backlash. When the drive train has backlash, it can move freely within the backlash region, the open region between the teeth. Backlash can lead to pointing/positional delays, positional errors, poor sequential control and/or timing and the like.
Prior art backlash adjustment methods are known which mount an idler gear on an adjustable bracket to provide adjustment control between mating gears. This prior art method accomplishes adjustment, but makes it necessary to position the center distances between the idler gear and both its mating gears.
Another prior art backlash adjustment method improves upon the positioning solution, set forth above, though the backlash adjustment is made only between the idler gear and one of its mating gears while eliminating the need for adjustment between the idler gear and its mating gears. This is accomplished through the use of a connector of fixed length. With the idler gear mounted on an adjustment bracket, the connector is attached to the idler gear and one mating gear establishing a fixed center distance between the two gears. Backlash adjustment can then be made between the idler gear and the other mating gear without affecting this fixed center distance provided by the connector. However, this prior art backlash adjustment method requires the use of both the connector and the adjustment bracket. The prior art backlash adjustment method further requires complicated machining of standard parts in order to mount and utilize the invention and other problems.
While the above mentioned prior art only use passive methods for eliminating backlash, there are also active prior art backlash compensation mechanisms. These systems use two motors and drive gears and one driven gear, the driven gear being coupled to the axis to be moved with highest precision and without backlash. These methods may include a torque based backlash control approach, limited to two motors per axis and utilizing a controller loop. A torque bias is added to a torque set point for one motor and subtracted from the other motor. The value of the torque bias depends on the total axis torque. Although the torque based backlash control system can compensate varying backlash almost independently of the rate of change of the backlash, the relative motor positions are uncontrollable and under load variations, significant beating between pinions and bearing gear flanks can appear.
Examples of such systems are described in U.S. Pat. No. 5,729,100, Rothstein et al., and in “Torque Bias Profile for Improved Tracking of Deep Space Network Antennas” and “Control System of the Array Antenna Test Bed” by W. Gawronski et al. The principles discussed in these references consist of applying different torques to the two drive gears such that one drive gear opposes the movement of the other and thus eliminates backlash. The micro-controller receives as an input the torque to be applied to the driven gear, which it distributes to the two motors, whereby a torque offset or bias is added to the one motor torque set point and the same offset or bias is subtracted from the other motor torque set point. In the Rothstein approach, an almost constant torque bias is maintained over the whole range of driven gear torques. In the Gawronski approach, the torque bias is dependent on the driven gear torque, where the bias is decreased at high loads, such as to obtain a more even distribution between the two motors. In both cases, the motor torques are controlled, the motor positions are not.
Therefore, a need exists for an improved backlash control system and method that provides improved precision positional control and motor load balancing.